Polysilicon produced by a fluid bed process

ABSTRACT

Silicon of high purity is made by decomposing silane in a fluidized bed reactor. To obtain good process economics, two modes of operation are used. In a first mode, the reactor is operated under high productivity conditions which also result in co-production of silicon dust or fines. The dust on the particles can cause problems in handling. For example, in bagging the particles, or removing the particles from a bag, the dust can become airborne from the larger particle surfaces and form an objectionable cloud of silicon dust. The invention provides a method for uniting dust to the larger silicon particles. In a second process mode, a thin (0.1-5.0 micron) layer of high purity silicon is deposited on the dust laden particles. This second mode is most preferably carried out by (a) treating the dust-laden particles with a deposition gas comprising 1 to 5 mole % silicon admixed with an inert carrier gas such as hydrogen, (b) in a fluidized bed reactor, and (c) at a process temperature of 620°650° C. The product polysilicon is composed of free flowing, approximately spherical particles having a size distribution of 150-1500 microns, an average size of 650-750 microns and has a particle bulk density of 2.3 grams per cubic centimeter, a bulk density of about 1360 kg/m 3  and a silicon surface dust content of less than about 0.08 wt. %.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of parent application Ser. No899,906 filed Aug. 25, 1986 now abandoned, for Fluid Bed Process andProduct.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to silicon produced by silane pyrolysis in afluidized bed reactor. More particularly, it pertains to an improvedform of high purity silicon.

2. Description of the Prior Art

As a base material for semiconductor devices, silicon is more widelyused than any other semiconductor. Silicon's dominant role results fromits unique, favorable combination of semiconductor properties.Throughout the world, semiconductor grade silicon is produced by theSiemens process. In that process, slim rods of silicon are heated byelectric current, and the heated rods are exposed in a suitable vesselto a gaseous mixture of hydrogen and trichlorosilane. Under the reactionconditions employed, silicon deposits on the rods, causing them to growin size. In a modification of the Siemens process, silane is used as agaseous source of silicon, rather than trichlorosilane.

Rods of polysilicon produced by the Siemens and modified Siemensprocesses described above, are not used directly. Instead, the rods aresawn or broken into chunks. The chunks are irregular in shape and aboutthe size of a man,s fist, or smaller. They are characterized by havingirregular faces bounded by sharp, irregular surfaces. The chunks are notfree flowing.

In order to make the solid state electronic devices used today, it isfirst necessary to transform polysilicon into monocrystalline siliconWorldwide, about 80% of this basic material is produced by theCzochralski method; the rest mainly by the float zone method. In 1984,about 2500 metric tons of single crystal silicon was produced by theCzochralski process. This represents over 73 billion semiconductordevices.

In the Czochralski process, polysilicon is melted in a suitablecrucible, a seed crystal is dipped into the melt, and then slowlywithdrawn exactly vertically to the melt surface. Liquid siliconcrystallizes on the seed. The result is an essentially single crystalrod with the angle between the cylindrical axis and the crystalorientation being close to zero.

The volume of the crucible employed cannot be totally filled bypolysilicon chunks. Depending on the size of the chunks there is anunfilled crucible volume of about 30-50% between the chunks. To minimizethe unfilled volume, chunks are piled above the top surface of thecrucible, and to do this well, the chunks are stacked by hand. Theoperator visually selects the size and shape of the chunks in order tostack enough to charge the crucible with the proper weight ofpolysilicon. This process is laborious and time consuming. Furthermore,the operator's hands and gloves are frequently lacerated by the chunkedges.

In contrast to the chunks described above, the polysilicon of thisinvention is composed of free flowing, approximately sphericalparticles. They can be transported and handled readily. For example,they can be automatically charged to the melt crucible without beingtouched by the operator.

As more fully set forth below, the product of this invention is made bya fluidized bed process. At least one company makes polysilicon for itsown internal requirements using a fluidized bed method in whichtrichlorosilane serves as the gaseous source of silicon. In contrast,the product of this invention is produced in a fluidized bed fromsilane. This reduces the opportunity or chances of chlorinecontamination of the present product compared to the prior art material.

Use of silane is a fluidized bed reactor is not without problems Siliconfines or dust is readily produced as a by-product. The dust is not onlyobjectionable, but it represents an economical loss as well. Someaspects of dust formation are discussed in the art.

Eversteijn, Philips Res. Repts. 26, 134-144, (1971) comprises a study ofgas phase decomposition of silane in a horizontal epitaxial reactor. Itwas found that gas phase decomposition is a serious factor that must betaken into account. In order to avoid gas phase decomposition, themaximum silane concentration in the hydrogen admitted to the reactor was0.12-0.14 volume percent, dependng on the gas temperature. When thiscritical silane concentration was exceeded, gas phase decompositionoccurred giving rise to silicon fines which deposited on the substrate.

The Eversteijn article is referenced in Hsu et al, J. Electrochem Soc.:Solid State Science and Technology, Vol. 131, No. 3, pp. 660-663,(March, 1984). As stated there, the success of the Siemens process ledto its universal adoption for producing semiconductor grade silicon, andthe de-emphasis of fluidized bed process development. In 1975, thepotential market for semiconductor grade silicon for photovoltaic usemade fluidized bed (FB) production of polysilicon more attractive.Fluidized bed operation has the capabilities of high-throughput,continuous operation and low energy cost. Because silane has a lowdeposition temperature, and can be completely converted in anon-reversible reaction, it is attractive for use in fluidized bed (FB)operation. Additional advantages are the non-corrosive atmosphere, andease of recycling by-product hydrogen. In conventional chemical vapordecomposition devices, there is a limit of silane concentration inhydrogen beyond which unwanted fines are homogeneously nucleated. Thus,in addition to the desired decomposition, silicon dust or fines appearin the gas phase. These particles vary in size from submicron to ˜10microns, and present mechanical problems in the operation of thereactor. They are also difficult to transport. Dust and fines areconsidered losses in the process. Hence, conventional reactors areoperated with low silane concentrations to prevent excess finesformation. In a fluidized bed reactor, less fines are generated because(i) there is less free space available for homogeneous nucleation and(ii) the silicon particles scavenge the fines and incorporate them intothe deposition growth. Consequently, the net amount of fines is lessthan for chemical vapor deposition apparatus, and a fluidized bedreactor can be operated at much higher silane concentrations withgreater throughput. Variables which effect the amount of fineselutriated were studied. Conclusions reached were as follows:

Elutriated fines increase with increased silane concentration, increasedtemperature, increased gas bubble size, and increased gas velocity. Theauthors selected 600°-800° C. and a gas velocity of U/U_(mf) =3-8 asgood operating parameters.

Another article, Hsu et al, Eighteenth IEEE Photovoltaic SpecialistsConference (1984) pp. 553-557, discusses additional studies on finesformation. It states that silane pyrolysis in a fluidized bed reactorcan be described by a six-path process: heterogeneous deposition,homogeneous decomposition, coalescence, coagulation, scavenging, andheterogeneous growth on fines. The article indicates that finesformation can be reduced by providing at a suitable bed location, asecondary source of silane for cementation.

The cited art clearly shows that production of silicon via decompositionof silane is complicated, and that provision of improved processes isnot straight forward. Nonetheless, because of continuing advances in theelectronics industry and the development of new products in that field,improvements in existing technology are needed to provide high puritysilicon at reduced cost. This improved product of this invention arisesfrom a process which enhances operation of fluidized bed methods, byproviding means to make high quality product under high productivityoperating conditions. The product is very pure, and has little dust. Itsphysical form is such that it is highly attractive to the siliconindustry. For these reasons it is fair to say that the product is highlyrevolutionary and an advance in the art.

SUMMARY OF THE INVENTION

This invention provides a polysilicon product characterized by beingfree flowing and in the form of approximately spherical particles havinga size distribution of from 150-1500 microns, an average size of 650-750microns, a particle density of about 2.3 grams per cubic centimeter anda bulk density of about 1360 kg/m³, said product being furthercharacterized by having a silicon surface dust content of less thanabout 0.08 wt. %. This product comprises dust and larger particlesbonded together. It is suitable for the production of monocrystallinesilicon for semiconductor devices.

The products of this invention are free flowing and can be automaticallyhandled with a monocrystalline silicon production facility.

In a preferred method, the products of this invention are made by aprocess comprising the step of intimately contacting:

(A) A bed of particles of high purity silicon maintained;

(i) in a vertically disposed reaction zone, and

(ii) at a reaction temperature higher than the thermal decompositiontemperature of silane with;

(B) Silane contained in a first and second decomposition gas, each ofsaid deposition gases being introduced into said bed of particles at aflow rate sufficient to maintain said bed in a fluidized state withinsaid reaction zone, said first deposition gas being introduced for afirst deposition period and being a mixture of about 10 to about 15 molepercent silane in hydrogen, said second deposition gas being introducedfor a second deposition period beginning substantially immediately afterthe termination of said first deposition period, and being a mixture ofabout 1 to 5 mole percent silane in hydrogen, said process being furthercharacterized in that said first deposition period is from about 2 toabout 5 times as long in time duration as said second deposition period.

This invention also comprises (I) conducting the process describedimmediately above and (II) subsequently recovering the larger-sizedsilicon particles that are formed in said reaction zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, is a schematic representation, partly in cross section and notto scale, of a fluid bed reactor and attendant equipment in accordancewith certain embodiments of this invention. This figure pertains to asemicontinuous process or method of this invention.

FIG. 2 is a schematic flow diagram, not to scale, depicting a morecontinuous operation of this invention. In contrast to the method andapparatus of FIG. 1, which comprise usage of one fluidized bed reactor,the embodiments depicted by FIG. 2 employ two such reactors; the productof the first reactor being fed to the other reactor so that the firstreactor can be operated in a continuous or nearly continuous manner.

FIGS. 3, 3A, 4, and 4A are photomicrographs showing differences insurface characteristics of prior art polycrystalline prepared by theSiemens process and the process of this invention.

FIG. 5 is a particle size distribution cure of a typical product of thisinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment of this invention is polysilicon productcharacterized by being free flowing and in the form of approximatelyspherical particles having a size distribution of from 150-1500 microns,an average size of 650-750 microns, a particle density of about 2.3grams per cubic centimeter and a bulk density of about 1360 kg/m³, saidproduct being further characterized by having a silicon surface dustcontent of less than about 0.0 wt. %.

Since the product is composed of small spheres, and is free flowing, ithas superior handling characteristics compared to chunks made fromSiemens polysilicon. Furthermore, as stated above, the free flowingcharacteristics obviate the laborious stacking of Siemens chunks into acrucible.

The products of this invention are made by a process comprisingdepositing on the surface layer of dusty silicon particles a thin layerof high purity silicon having an average thickness of from about 0.1 toabout 5 microns to cement silicon dust to the surface of said highpurity silicon particles, said layer being produced by the thermaldecomposition of silane gas, said process being conducted:

(A) By fluidizing a bed of high purity silicon particles havingremovable silicon dust on surfaces thereof with a stream of depositiongas having a motive force sufficient to maintain said bed in a fluidizedstate, such that such silicon particles are intimately contacted withsufficient silane contained within said deposition gas to deposit saidthin layer of silicon on said particles, said deposition gas consistingessentially of silane and an inert carrier gas admixed therewith, andcontaining from about one to about five mole percent silane;

(B) And at a reaction temperature between the thermal decompositiontemperature of silane and the melting point of silicon.

As indicated above, the polysilicon product of this invention is made ina fluidized bed reactor. Reference to FIG. 1 will illustrate thepreparative process.

The products of this invention have the commercially attractivecombination of high quality and free flowing form. With thiscombination, the products offer crystal growers for the first time, aproduct that is tailor-made for the development of continuous meltreplenishment systems.

The free flowing form of the products enhance their ability to bepackaged in containers which facilitate handling in the crystal grower'splant. For example, the free flowing particulates of this invention areconveniently packaged in bags. The polysilicon can be poured from thebags into a crucible for melting while the crucible is in place in thepuller device. Because the bulk density of the particulate product isapproximately the same as Siemens material, similar charges can be made.

In addition to bags, larger samples of the free flowing polysiliconparticulates can be shipped in appropriately coated drums which containup to 250 kg of product. The following examples illustrate handling ofthe particulate product.

EXAMPLE A

Small samples of approximately 10 kg of polysilicon product of thisinvention are packaged in a Teflon bag that is enclosed in twopolyethylene bags which, in turn, are packaged in a cardboard box (onesample per box).

Before opening, the plastic bags should be removed from the box in orderto avoid contamination with dust and box fibers. The sealed flap is cutand the outer polyethylene bag is removed. The second polyethylene bagis opened and the Teflon bag unfolded. The second polyethylene bag isnot removed from the Teflon bag which is designed to prevent tearing ofthe relatively fragile Teflon bag.

When pouring the polysilicon into a quartz crucible for melting, the baglip should be placed well down in the crucible.

EXAMPLE B

Stainless steel drums with a non-contaminating coating are used forbatches or samples of up to 250 kg of polysilicon product of thisinvention. The drums are thoroughly purged with inert gas prior tofilling with the polysilicon product. The container should be opened ina clean area and the time of exposure minimized. The polysilicon can beremoved with a quartz or silicon scoop or other non-contaminatingdevice.

For small charges, the crucible can be filled to the desired level,covered and then placed inside the puller. With large charges, it may benecessary to partially fill the crucible, place it in the puller andthen add the remainder of the charge in a noncontaminating manner.

A conical top which includes an integral flow control device can be usedfor multiple drum handling. The following procedures are recommended foremptying drums of polysilicon product of this invention using thisdevice:

1 Vacuum the top of the drum and the conical top assembly to removeparticles and dust.

2. Remove the drum cover and securely install the conical top on thedrum.

3. Using a mechanical drum inverter, invert the drum assembly and placeover a crucible loading station.

Note: A crucible loading station should consist of a small booth withshields or curtains and a HEPA filtered air system or other means ofproviding a clean environment for crucible loading.

4. Place crucible or other noncontaminating containers under the drumand open the flow control valve to regulate polysilicon loading.

5. Cover the loaded container and transport to the furnace.

6. Repeat this procedure until the drum is empty.

7. Invert the drum, remove the conical flow valve assembly and securelyfasten the original cover to the drum.

Free flowing particulate polysilicon of this invention has beensuccessfully melted in several different crystal growing systems over arange of charge sizes. These results demonstrate that polysilicon ofthis invention offers the semiconductor and solar industries anattractive alternative to material currently used in Czochralski typepullers.

Some melting procedures which hold the crucible high in the heatingzones have resulted in occasional bridging of the surface pellets. Thisappears to be associated more with large charges where the surface ofthe charge is insufficiently heated. For smaller charges (20 kg or less)many conventional procedures work quite well with particulatepolysilicon.

One method that has proven very successful in providing adequate heatingto the surface of large charges involves the use of a heat reflectorduring the melting cycle. A thin flat disc or conical shaped molybdenumreflector is suspended from the seed lift mechanism about 3-5 inchesabove the surface of the charge. The charge is then melted using aconventional melting procedure; starting with the crucible high andlowering in increments after a given time or event. After the charge hasmelted and is being stabilized, the reflector is isolated in the pullchamber, removed from the seed lift mechanism and replaced with the eed.Specific procedures utilizing heat reflectors that have worked quitewell for different charge sizes and pullers follow.

EXAMPLE C Melting Polysilicon of this Invention

Puller: Hamco 3000, Analog controls

Crucible 14" diameter

Charge: 28 kg polysilicon of this invention

Heat Reflector: 61/2" diameter, conical shape (30° cone)

Install the molybdenum reflector on the seed lift mechanism and lower to3 inches above the surface of the charge. (Use maximum diameter that canbe removed through isolation valve.)

Start with the crucible high (+2.0") and rotating at 1 RPM.

Control the heat imput by maintaining a constant temperature in the hotzone about 100° C. lower than that used for rod polysilicon.(Thermocouple or pyrometer in hot pack insulation).

After 80 minutes lower the crucible to -2.25 inches (jog down) and lowerthe reflector to 2 inches above the heater.

After an additional 40 minutes raise the crucible to -2.0 inches.

When the charge slips into the melt, raise the temperature by about 100°C. to the temperature used for melting rod polysilicon and finish themeltdown.

When the entire charge is molten and clean, reduce the temperatureand/or heat imput to that normally used for stabilization.

While the melt is stabilizing, isolate the reflector in the pullchamber. (Be careful that reflector does not catch on the throatopening.)

Remove the reflector, install the seed and evacuate the pull chamber.

When the pressures have equilibrated, open the isolation valve andprepare for seed dip.

The remainder of the cycle should be the same as used for conventionalmaterial.

EXAMPLE D Melting Polysilicon of this Invention

Puller: Hamco 6000, Analog controls

Crucible: 14" diameter

Charge: 28 kg polysilicon of this invention

Heat Reflector: 91/2" flat dual disc separated by 1/2" air space

Install the molybdenum reflector on the seed lift mechanism and lower to5 inches above the surface of the charge. (Use maximum diameter that canbe removed through the isolation valve.)

Start with the crucible +0.5 inches above the heater and rotating at 1RPM.

Control the heat input at that used for rod polysilicon during theentire melting cycle (approximately 110 kw. for this machine).

After 30 minutes, lower the crucible to -0.5 inches. (Do not movereflector.)

After an additional 30 minutes, lower the crucible to -1.5 inches. (Donot move reflector.)

When the entire charge is molten and clean, reduce heat rate to thatnormally used for stabilization.

Stabilize melt for about an hour.

While the melt is stabilizing, isolate the reflector in the pullchamber. (Be careful not to catch on the throat opening.)

Remove the reflector, install the seed and evacuate the pull chamber.

When the pressures have equilibrated, open the isolation valve andprepare for seed dip.

The remainder of the cycle should be the same as used for conventionalmaterial.

With regard to melting parameters for large charges, experience to dateteaches that the melt should begin with the crucible high to initiallyheat the bottom of the charge as is done with conventional polyilicon.The crucible should then be lowered to 2 to 3 inches below the top ofthe heater for some time period during the first half of the meltingcycle to heat the surface and prevent severe bridging. When the chargeslips into the melt, the crucible can be adjusted to a conventionallevel for the remainder of the melt. It has been found that holding thecrucible low in the hot zone for long periods or using high heat ratesduring the first half of the melt cycle can cause excessive oxideformations, which sometimes results in loss of structure during crystalgrowth. The heat balance without a reflector appears to be moredelicate, and different combinations of crucible elevation changes andheat rate adjustments will likely be required for different pullingsystems and charge sizes to optimize performance.

Other parameters such as crucible rotation, inert gas purge rate, systemvacuum, etc. for melting polysilicon of this invention should be similarto those used for conventional material. The initiation of crystalgrowth and the remainder of the growing process should be the same asused with conventional polysilicon.

In FIG. 1, a fluidized bed reactor is illustrated by reactor 10 having areaction zone containing particles of high purity silicon. The reactoris fitted with external heating means 12 surrounding the reactor zone,and sufficient to heat the bed of particles to an operation temperatureabove the decomposition temperature of silane. The reactor is fittedwith feedline 14 for introduction of seed silicon particles, and line 16for removal of larger-sized silicon particles formed by the process ofthe invention. The reactor has a gas distributor 18 which is a multiplepore device through which silane and hydrogen, or other carrier gas, isintroduced into the bed of particles in the reactor. The pores of thedistributor device are numerous, to facilitate intimate contact of thedeposition gases with the particles in the reaction zone. Line 20provides for exit of gases, (such as carrier gas, unreacted silane, andby-product hydrogen) admixed with silicon fines or dust particles formedduring the process. The fines are removed by collection device 22.Hydrogen from the collection device can be recycled through line 4, pump26 and a heat exchanger, not shown, back into the reactor as carriergas. The process is a net producer of hydrogen and therefore a portionof hydrogen from device 22 can be sent via line 28 to other uses, notpart of this invention.

The hydrogen and silane are admixed and adjusted to desiredconcentrations by passage through metering and monitoring devices (notshown). After admixture, the resultant gas mixture enters the reactordevice below distributor 18 via line 30. The incoming gas may bepreheated if desired. To prevent fouling and plugging, distributor 18may be fitted with cooling means, (not shown).

Preferably, the reactor is first operated under high productivityconditions, for example by using a deposition gas containing 10 to 100mole percent silane and a process temperature of preferably 620°-750° C.When operated in this manner, a significant amount of silicon metal dustor fines is formed as a co-product. Some of this is removed by line 20and collected in collection device 22, as described above. Other dustparticles are deposited on the surface of the larger silicon particlesin the fluidized bed, and can cause problems on handling after thelarger particles are removed from the device.

To avoid these problems, the operation of the device is shifted to amode of operation which adheres the dust particles or fines and thelarger silicon particles in the bed. This mode comprises reducing theamount of silicon dust in the product by fluidizing the particulateproduct including silicon dust with a gaseous stream comprising avaporous silicon-containing compound and maintaining the temperature tofavor chemical vapor deposition of silicon so that a thin layer ofmetallic silicon is formed on the particulate product and silicon dustis adhered thereto. Thus the preparative method of this inventioninvolves two depositions of silane.

Thus in the process employed, a significant amount of the surface dustdeposited on the particle surfaces during the high productivityoperation will have become glued on, bonded or united, i.e. made toadhere to the larger particles by the thin layer deposited from thesecond deposition gas. The dust or fines are typically composed ofparticles about 0.2-0.5 microns in size; the particles on which thesilicon deposition take place preferably have a surface mean diameter ofabout 400-1000 microns, more preferably about 400 to about 700 microns.Generally, during the second deposition, homogeneous decomposition ofsilane cannot be entirely avoided and consequently some additional dustis deposited upon silicon particles in the bed. However, since thesecond deposition is conducted under conditions which heavily favorheterogeneous deposition, the amount of additional dust deposited isrelatively small. However, some dust may remain on the particles afterthe cementing operation and may be produced by that operation.

The use of two deposition gases in the invention as discussed abovereflects two important aspects of this invention. First, it isdesirable, from the standpoint of productivity, to operate a fluidizedbed reactor for production of silicon from silane under conditions bywhich silicon is deposited rapidly, but which (unfortunately) also causehomogeneous decomposition of silane to occur to a significant extent,thereby resulting in formation of a significant amount of silicon finesor dust coproduct. Although the fines or dust elutriated from thereactor are not nearly as valuable as the larger silicon particlesproduced, and in fact can amount to waste, operation in such a regime isattractive because growth of the silicon particles occurs at a ratefaster than achievable under conditions where only chemical vapordeposition takes place. To upgrade the product particles, it isdesirable to lower the amount of dust that is loosely deposited on thesurface. Of course, the dust can be removed by polishing or by immersingthe dusty particles in a liquid and agitating the resultant mass, saywith ultrasonic vibration to assist removal of the dust. But analternative method is needed since such treatments are costly, wastefuland can introduce a source of particle contamination. This inventionsatisfies that need.

The second important aspect comprises the discovery that the use of thesecond deposition gas as taught herein comprises a treatment thatcements surface dust, resulting in a product that not only has therequired purity, but which can be more readily handled. It was alsounexpectedly discovered that this bonding or uniting of the surfaceparticles requires less than one micron, for example 0.1-1.0 microns.Hence, for this invention 0.1 to 5.0 microns of additional silicon aredeposited on the particle surface.

In FIG. 2, reactor 50 is a fluidized bed reactor substantially asdescribed above and as depicted in FIG. 1. The reactor is charged with abed of silicon seed particles from line 52 near the top of the reactor,as shown. Under the selected reaction conditions, the bed is fluidizedand the particles therein contacted with the first deposition gas. It ispreferred that this first gas be a mixture of SiH₄ and H₂ (designated by"FIRST SILANE/H₂ GAS" in the drawing). This gas is introduced into thereactor via line 54. In the reactor, deposition of silicon anddecomposition of silane occurs, resulting in growth of the siliconparticles, deposition of silicon fines on the surface of said particles,and elutriation of additional fines with carrier gas and by-producthydrogen through exit line 56 at the top of the reactor. After theparticle growth, a portion of the bed of particles is taken off by line58 near the bottom of the reactor. This is replaced in the first reactorwith additional seed particles and the first deposition in that reactorcontinued.

Line 58 is connected to fluidized bed reactor 60 such that particleswith surface silicon dust are transferred from reactor 50 to reactor 60.In reactor 60 the second deposition gas, i.e. designated "SECONDSILANE/H₂ GAS" in the drawing is introduced through line 62. Aftercementing dust on the particle surfaces, product particles are removedthrough line 64. Gas and any elutriated fines exit the reactor throughline 66. Hydrogen produced in either or both reactors can be recycled toeither or both reactors after admixture with silane, or sent on forother usage.

In this embodiment the second reactor can be smaller than the firstbecause only a minor portion of the total silane is contacted with theparticles in the second reactor. It can also be operated at a differenttemperature.

Although from a theoretical point of view it might be better to operatesuch that all silane contacted with the bed of silicon particlesdecomposes to form silicon while being contacted with the bed, this isgenerally not the preferred case in actual practice. It has been foundadvantageous to operate the first step, i.e. the high productivity modeunder conditions wherein about 10-25% of the silane does not react andexits the reactor. Operation in a regime that comprises this featureenhances productivity; growth or deposition rates are higher at higherconversions, however dust production is high when conversions are high.For the second mode dust formation is much reduced.

Generally, there is a threshold or minimum gas velocity required to keepthe particle bed in a fluidized state. Operational velocities for inputof deposition gas into the bed are generally somewhat above thisminimum, U_(min). In many instances the operation velocity U, is 1 to 10times U_(min) ; preferred velocities are 1.2 <U/U_(min) <8; morepreferably, 1.5 <U/U_(min) <3.5.

In the process of this invention, the first and second deposition gasmay be introduced at the same or different rates, as desired. Generally,good results are obtained if the hydrogen or other inert gas isintroduced at about the same rate, and differing rates are used forsilane in order to adjust the concentration of silane. As indicatedabove, the first deposition gas is preferably used so that silane isintroduced at a rate which together with the other variables favors highproductivity, while the second gas is introduced with silane underconditions which heavily favor chemical vapor deposition and reduce theamount of homogeneous decomposition. In many instances the slowerintroduction of silane in the second step or mode results in a higherpercent of silicon introduced (as silane) being deposited.

For this invention, the introduction of gases into the fluidized bed isconducted by introducing the gases at a slightly positive pressure tofacilitate fluid flow. The pressure of the gas introduced at or near theinterface of the distributor and the bed is generally 1 to 3atmospheres, more preferably from about 1.01 to 2 atmospheres.

Intimate contacting of the gaseous reactant and deposition surface isfacilitated by introducing gas into the bed through a distributor havinga plurality of openings in its surface adjacen to the bed of particles.Preferably the openings are substantially uniform, relatively small, andsubstantially evenly spaced across the surface that is adjacent to thebed surface.

As immediately recognizable by a skilled practitioner, it is necessarythat the process be conducted above the decomposition temperature ofsilane; i.e. above about 500° C. Thus, suitable means must be providedso that the deposition gas being contacted with the silicon particles isabove the temperature at which silane thermal decomposition begins totake place. The process temperature is further selected so that therelative rates of (a) deposition of silicon on the particle surfacescompared to the (b) rate of fines formation via homogeneous gas phasedecomposition is within acceptable limits. Thus, it is preferred thatthe process temperature be within the range of from about 590° C. toabout 650° C.; more preferably from about 620° C. to about 650° C. Thetemperature can be any temperature between the thermal decompositiontemperature of silane and the melting point of silicon. The preferredtemperatures given above are selected for use with silane and by suchconsiderations as the level of impurities picked up from feed lines inthe reactor employed, and the degree of homogeneous decomposition.Utilizing a different system or a different degree of homogeneousdecomposition, the process can be operated very well with differentpreferred temperatures. Usually the temperature for the first and secondmode are about the same since it is inconvenient to change thetemperature because of the high heat capacity of the apparatus;especially when it is comparatively large in size.

To facilitate maintaining the desired temperature in the reaction zone,the gases used for silicon deposition and/or to maintain the particlebed in ebullient motion can be preheated prior to introduction into thereactor. For example, the hydrogen can be preheated. Preheating can beto some temperature level below that which causes silicon depositionwithin the distributor. To help avoid this diffioulty, the distributorcan be fitted with cooling means. Moreover, the gas should not be heatedso high as to cause an untoward amount of deposition near thedistributor which welds or solders so many beads together that anuntoward amount of pluggage occurs. It has been found that good resultsare obtained if the gas is preheated to a temperature of about 300°-400°C.

The process of this invention is conducted using a fluidized bed ofsilicon particles. These particles are of sufficient purity to beacceptable for the use intended. The seed particles used to prepareparticles in the bed can be prepared by this invention followed byreducing particle size to an average of 200 microns with an 80-400microns range. Seed particles can be irregular in shape. They tend tobecome substantially spherical during operation of the reactor.Preferably the bed particles after silicon deposition have a d_(ps) of400-1000 microns, more preferably from about 600 to about 800 microns.However, beds having a d_(ps) of 300-2000 microns can be used. Theaverage particle size and the size range is not critical, so long as thebed can be fluidized under acceptable operating conditions.

The process of this invention can be operated in the high productivitymode for as long as desired. In other words, for the high productivityoperation, time is essentially an independent variable and is onlygoverned by convenience, reactor capacity, amount of silane available orsome similar operation variable or variables. As an example, when usingan 18" reactor, at a productivity rate of 50 pounds of silicon per hour,a reaction temperature of 650° C., a deposition gas comprising 12-14%silane in hydrogen, a bed of particles weighing about 350 kg and havingan average particle diameter of 450 microns (mu), it is convenient tocease high productivity operation when the bed weight has increased byabout 40 kg to total of about 390 kg.

a

After about that increase in weight, operation is shifted to the qualitymode to unite dust particles deposited on the surface of the particlesin the fluidized bed and thereby prepare the product of the invention.Typically, the quality mode for an 18" diameter reactor comprisescontacting the bed of silicon particles with deposition gas of 1-5,preferably 2-4 mole percent silane in hydrogen, for the time required todeposit an additional layer of about 0.1 to about 5 microns inthickness. This causes a significant part of the dust on the particlesto glue on to the particles and form the improved product.

After the second layer is deposited, product is discharged from thereactor. Generally one removes about the weight of (a) new seedparticles introduced plus (b) the weight of silicon deposited during theproductivity and gluing on cycle.

To achieve preferred results, the use of the second deposition gas isconducted for a relatively short time but sufficient to cause adiminishment of the amount of readily removable dust on the surface ofthe silicon particles. Generally it is preferred to keep the duration ofthe quality-mode relatively short, so that operation can relativelyquickly return to the high productivity mode and thereby allow theprocess to be conducted at a high overall productivity rate.

The general desirability to keep usage of the second deposition gas to ashort duration has two ramifications. First, it is generally preferredto select as thin a second coat as will effectively do the job. As shownin one example given below, after about the first micron in thickness,additional deposition did not have an appreciable effect in reducingdust. Second, since the amount of silane to be utilized in the seconddeposition is a quantity that can be fairly closely estimated bycalculation, and since the concentration of silane in the seconddeposition gas must be comparatively low so as to operate in a regionwhere chemical vapor deposition is highly favored, the time durationwhen the second gas is employed is a dependent rather than anindependent variable.

Generally, good results are obtained when the elapsed total time, whichis the sum of the time durations of the first and second depositionperiods, is within the range of about 2.5 to about 5 hours. Also, goodresults are obtained when the first deposition is from about 2 to about5 times as long in duration as the second deposition. It is preferred touse a process sequence where the first deposition period is from about21/2 to about 31/2 hours in duration and the second is from about 1/2 toabout 11/4 hours in operation.

Preferably, the second deposition gas flow is begun before 10 minutes orso has elapsed from the termination of treatment with the first gas,i.e. the second gas is contacted with the silicon particlessubstantially immediately after the first gas has been contacted.

It is not necessary that the process be terminated after the second stepis conducted. The sequence of the first and second deposition can berepeated after removal of product from the reactor and the addition ofseed particles. For example, with product removal and replenishing ofseed particles as indicated above, the process can be run one or twoweeks or more, i.e. indefinitely, by repetition of the depositionsequence 100 or more times.

To conduct the process of this invention, the operator charges thereactor with the desired amount of silicon bed particles. The reactorvolume filled with particles is measured. After using a first depositiongas, the mass of silicon that has been deposited within the particle bedis determined. For example, the operator may charge the reactor with 300kg of bed particles having an average particle size or surface meandiameter (d_(ps)) of 625 microns. After bringing the bed to an operatingtemperature of say 640° C., a first deposition gas, for examplecomprising 65 standard cubic feet per minute of hydrogen and 50 poundsper hour of silane, is introduced into the reactor and this gas mixtureis continuously fed for three hours. From reactor effluent gas analysisfor unreacted silane, and from the amount of dust collected in theeffluent gas during the feeding period, the operator can determine whatpercentage of silane fed has deposited silicon on the bed particles. Forexample, assume the operation results in 90% of the silicon that was fedas silane being deposited on the bed particles. Then: ##EQU1## This isequal to (39.4÷2.2) or 17.9 kg/hr of silicon deposited. Therefore, theincrease of bed weight over 3 hours is (17.9×3) or 53.7 kg. The increasein bed weight can be expressed as (54.7/300) or 18%. From a sample ofthe bed particles, the operator determines the particle sizedistribution using a screen analysis. From this determination theoperator calculates surface area per gram or surface mean diameter,d_(ps). For example, assume screen analysis shows that the d_(ps) afterthe three hour deposition has increased from 625 to 650 microns. Then,the total bed particle surface area is given by the expression: ##EQU2##where ρ is the density of silicon, i.e. 2.32 gm/cm³. Hence, Ap in thisinstance will be: ##EQU3## Assume further that the operator wishes todeposit a layer of silicon of average thickness (Δ) or (Δx) of 1.5micron to make the dust on the surface of the particles adhere to andbecome a part of the particles. Then, the amount of silicon to bedeposited is given by the relationship: ##EQU4## If the operator assumesa 95% deposition efficiency for the silicon fed as silane, then thetotal amount of silane to be fed in the second deposition gas will be##EQU5## If the silane is to be provided in a concentration of 4 molepercent with hydrogen introduced at 65 SCFM, then the followingcalculations show the rate of introduction of silane. ##EQU6## Thisamount of hydrogen is equal to 9828/2 or 4914 moles of hydrogen perhour. At a desired concentration of 4 mole percent in the seconddeposition gas, (0.04×4914) 0 96 or 204 moles of silane must be fed perhour, i.e. or 6.55 kg of silane. Since only 5.9 kg of silane is requiredfor depositing the 1.5 micron layer, the operator will feed the silaneat the calculated rate for 0.9 hours, i.e. 54 minutes.

The total depositions are 53.7 +4.9 or 58.6 kg of silicon. Hence theoperator can withdraw 58.6 kg of product from the reactor, calculate thenumber of product particles and replenish the reactor with the samenumber of seed particles and repeat the cycle.

Table I gives surface dust measurements of typical fluid bed operation.The data were obtained from three different size reactors configured asshown in FIG. 1 and operated as described above. Particle size, silaneconcentrations, and bed temperature were varied as indicated in thetable. These data show a direct relationship between the reactorproductivity and the amount of dust adhering to the particle surface. Toproduce polysilicon with an acceptable surface dust level (e.g., 0.1 wt.%) with a reactor operated sing typical prior art fluid bed operatingprocedures, reactor productivity would be limited to 10-20 lbs/hr ft².However, because of obvious economic considerations, it is desirable tooperate at higher productivities and, much higher productivities areusable. Examples of higher productivities are given in the table. Thusdust produced by the high productivity mode exemplified by Table I canbe reduced in amount following the procedures of the gluing on modedescribed herein and illustrated by the examples.

                                      TABLE I                                     __________________________________________________________________________    PRODUCT SURFACE DUST FOR TYPICAL FLUID BED OPERATION                                     Average                                                                             Silane Feed Average Bed                                           Reactor                                                                             Bed Temp.,                                                                          Productivity,.sup.1                                                                  Molar %                                                                            Particle Size                                                                         Mass,                                                                             Surface                              Reactor                                                                            1.D., inch                                                                          °C.                                                                          Lb/Hr Ft.sup.2                                                                       In H.sub.2                                                                         (dps).sup.2, micron                                                                   kg  Dust, Wt %                           __________________________________________________________________________    1    4.5   650.  11.    7.5              .08                                  2    6.25  650.  33.8   12.  777.    52. .22                                       6.25  650.  36.1   12.  714.    50. .33                                       6.25  650.  44.6   14.  748.    31. .34                                       6.25  650.  33.3   14.  660.    50. .34                                       6.25  650.  33.3   14.  690.    50. .36                                       6.25  650.  46.9   18.5 724.    45. .48                                  3    14.5  660.  19.-25.                                                                              5.7-7.5                                                                            845.    370.                                                                              .092-.123                                 14.5  645.  44.    15.4 545.    370.                                                                              .31                                       14.5  627.  44.    15.4 328.    230.                                                                               .197                                     14.5  624.  44.    12.1 851.    220.                                                                               .358                                     14.5  624.  44.    13.0 671.    280.                                                                               .284                                     14.5  632.  65.    19.1 627.    230.                                                                               .221                                __________________________________________________________________________     .sup. 1 Productivity defined as the silane feed rate per reactor cross        sectional area.                                                               .sup.2 Surface mean diameter, microns.                                   

EXAMPLE 1

A 400 kg bed of silicon particles was charged to the 14.5 inch diameterreactor and operated for 80-90 hours in a semicontinuous mode wherebyseed particles were added every 1-2 hours and product was removed every1-2 hours. By this procedure, bed level was maintained essentiallyconstant during this period. The bed was subjected to depositionconditions as follows:

Average bed temperature: 645° C.

Silane feed rate: 25 lbs/hr

Hydrogen feed rate: 23-33 scfm

Silane feed concentration: 11.5-15% molar

U/U_(min) : 2.2-3.2

Silicon Deposition: 19 lbs/hr

At the end of the period a sample was taken for screen and surface dustanalyses. The surface mean particle diameter, d_(ps), was determined tobe 460 micron and particle surface dust was 0.198 wt. percent.

To analyze for surface dust, a 10 gram sample of silicon particles wasplaced in 10 ml of methanol in a screw capped bottle (approx. 4 oz.capacity) and placed in a water bath of an ultrasonic shaker device andsubjected to ultrasonic vibrations (nominally 55,000 vibrations persecond) for 30 minutes. The methanol with silicon dust particlessuspended therein was passed through a sieve of 125 mu mesh. Theprocedure Was repeated until the methanol remained clear aftersonification. The methanol/silicon dust portions were combined andevaporated to dryness. The weight of the dried dust removed wasdetermined. As indicated above, after the first deposition the weight ofthe dust was 0.198 wt. percent of the sample.

To glue surface dust on to the larger particles the bed was subjected tosecond deposition conditions as follows:

Average bed temperature: 645° C.

Period: 0.5 hours

Silane feed rate: 4 lbs/hr

Hydrogen feed rate: 31 scfm

Silane concentration: 2.5% molar

Silicon deposition rate: 2.8 lbs/hr

A sample of the particles was analyzed as above and the result was 0.2wt. % surface dust. The silicon deposited was sufficient to add layer ofabout 0.1 micron thick to the bed particles. a

The procedure of this example can be repeated with the first depositionperiod using a silane feed stock containing 20%, 40%, 60%, 80%, or 90%silane admixed with hydrogen. Pure silane can also be employed. Thetemperature employed can be from 590° C. to 750° C. The gases used toglue on the surface dust can contain 1-5 mole percent silane inhydrogen. The deposition gases in both stages of operation, i.e. thefirst or high productivity stage, and the second mode, i.e. the gluingon process, can be preheated to 300°-400° C. prior to introduction intothe bed of particles. The particle bed can be maintained in a fluidizedstate by introducing the gases at a rate defined by U/U_(min) of from1.5 to 3.5. The deposition in the second mode can be conducted todeposit a layer of silicon of 0.1 to 5 microns in thickness. In thoseinstances where the gases contain hydrogen admixed with silane, thehydrogen utilized can comprise hydrogen recovered from the reactor andrecycled to the input gases.

In the process of the above example, the first deposition can beconducted by adding seed particles every 1.0 to 3.5 hours. The seconddeposition or gluing on step can be conducted over a period of 1/2 to11/4 hours in duration.

In the process of the above example, the deposition gases comprisingsilane and hydrogen, or substantially pure silane, can be introduced ata pressure of slightly above atmospheric pressure, i.e. about 1.01atmospheres to about 3 atmospheres.

In the process of the above example the gluing on step is preferablyconducted at 620°-650° C. using a gas containing 1-5, more preferably2-4 mole percent silane in hydrogen.

The process of the above example can be repeated using silicon particleshaving a d_(ps) of 400-1000 microns and with dust particles of fromsub-micron size, e.g. 0.2-0.5 microns up to about 10 microns. Theprocess of the above example can be repeated using seed particles of 200microns d_(ps) with a d_(ps) range of 80-400 microns.

EXAMPLE 2

Following the operation of Example 1, product was drawn from the reactorso that 370 kg of particles remained in the bed. The deposition ofsilicon was resumed and conducted for 24 hours using the followingconditions:

Average bed temperature: 645° C.

Silane feed rate: 50 lbs/hr

Hydrogen feed rate: 55 scfm

Bed level reduced to: 370 kg

A sample of the bed was removed to determine particle size distributionand surface dust:

dps 545 microns

Surface dust: 0.31 wt. %

The operation was resumed using a silane feed rate of 11.6 lbs/hr, and asilane concentration of 4% in hydrogen with a silicon deposition rate of8.1 lbs/hr. The effect of surface dust was determined over timeintervals of 1, 2, and 3 hours of treatment with the second depositiongas. The results were as follows:

    ______________________________________                                        Hours of                    Cementation                                       Treatment with   Surface Dust                                                                             Layer Added,                                      Second Deposition Gas                                                                          Wt. %      Microns                                           ______________________________________                                        0                0.31       --                                                1                0.072      0.91                                              2                0.063      1.81                                              3                0.073      2.72                                              ______________________________________                                    

The dimensions of the cementation layers were calculated from the 8.1lbs/hr silicon deposition rate, the 370 kg bed and the 545 micron dps.The discrepancy in surface dust weight percent for the two hour run isbelieved to be due to experimental error.

EXAMPLE 3

Table II below summarizes results and conditions of several typical longduration fluid bed operations during which the semicontinuous method ofthis invention was demonstrated.

Typically, a bed of silicon particles of desired particle sizedistribution taken from previous operation is charged to the 14.5 inchdiameter reactor. After desired bed temperature is reached, firstdeposition gas is fed for three hours. For each of the runs given inTable II, first deposition gas consisted of 50 lbs/hr silane mixed with70 scfm of hydrogen (12.3% silane). During this period, bed increasesfrom about 270 to about 315 kg of silicon.

At the end of the first deposition period, adjustments to silane andhydrogen feed rates are made to give the desired second deposition gascomposition. Silane feed rate is reduced to 14.1 lbs/hr and hydrogenrate increased to 78.3 scfm (3.4% silane). The second deposition gasfeed is continued for 1 hour which deposits about 4.5 to 5 kg of siliconessentially uniformly over the entire bed particle surface. Thisoperation is intended to deposit from 1.5 to 2.0 microns of surfacelayer depending on the exact particle size distribution.

During the last 10 minutes of the second deposition period, product iswithdrawn from the reactor to bring bed back to the level at the startof the cycle. The amount withdrawn each cycle is estimated by theoperator to be the sum of the weight of seed particles added plus thecalculated amounts of first and second silicon depositions. Once productis withdrawn, silane and hydrogen flows are reset to the firstdeposition conditions.

Seed particles are added to the reactor at the start of each cycle tomaximize growth on seed particles. Over the duration of the run, productparticle size is controlled by the operator by the number of seedparticles added each cycle. No attempt is made to change temperaturefrom first to second deposition period because of the slow response timeof this high heat capacity system. However, as feed gases are changed,bed temperature does increase slowly about 5° C. during the seconddeposition period and return slowly during the first. Table II gives theaverage temperature over the entire cycle.

                                      TABLE II                                    __________________________________________________________________________                              Product                                                                 Avg.  Size Surface Dust, Wt %                                                                             Layer                         Run                                                                              Hours of                                                                            No. of                                                                             Bed   Bed   (dps).sup.1,                                                                       No.           Std.                                                                             Thickness                     No.                                                                              Operation                                                                           Cycles.sup.2                                                                       Weight, kg                                                                          Temp., °C.                                                                   microns                                                                            Samples                                                                            Range                                                                              Mean                                                                              Dev.                                                                             Microns                       __________________________________________________________________________    1  172   43   270   650   662  13   .031-.127                                                                          .079                                                                              .027                                                                             1.6                           2  168   42   271   668   708  10   .012-.072                                                                          .052                                                                              .017                                                                             1.7                           3   68   17   247   656   698   3   .046-.06                                                                           .052                                                                              .007                                                                             1.9                           4  151   38   260   628   727  10   .022-.066                                                                          .042                                                                              .0151                                                                            1.8                           5  147   37   262   632   671  11   .015-.044                                                                          .030                                                                              .0083                                                                            1.7                           6  170   43   265   640   744  14   .016-.061                                                                          .039                                                                              .0151                                                                            1.9                              876                         61        .049                                 __________________________________________________________________________     .sup.1 dps is the surface mean diameter.                                      .sup.2 The cycle was fixed at:                                                1. 3 hrs at 50 lb/hr silane feed, 69.5 SCFM H.sub.2, then                     2. 1 hr " gluing on" at 14.1 lb/hr silane, 78.3 SCFM H.sub.2 product          removed last 10 minutes.                                                 

Table II shows that surface dust was reduced to the range 0.03-08 wt.percent practicing this invention. For the same productivity andtemperature range, dust would otherwise be between 0.3 and 0.35 wt.percent, Table I. Second deposition layer thickness ranged from 1.6 to1.9 microns during the six runs for bed weights in the 247 to 271 kgrange and surface mean particle sizes of 662 to 744 microns. These runscomprise a total operating time of 876 hours and 61 samples taken forsurface dust analyses.

The process of the above example can be modified and repeated so thatthe cycle used is a first deposition period of 2.5-3.5 hours and thesecond deposition period "gluing on" is 0.67-1.25 hours.

It will be apparent to a skilled practitioner that the procedure of theabove examples can be modified to a more continuous basis using two FBreactors. For example, the first step or high productivity mode isconducted in the first reactor, say by using a 400 kg bed of siliconparticles, a bed temperature of 645° C., a silane feed rate of 25lbs/hr, a seed addition rate of 1 lb/hr, a hydrogen feed rate of 27.5cfm, a silane feed concentration of 15% molar, and a U/U_(min) of2.2-3.2 and a silicon deposition rate of 19 lbs/hr.

Each hour, a 20 lb (19 +1) charge of particles can be transferred fromthe first to the second reactor and surface dust glued on using adeposition temperature of 645° C. The bed size is selected so that theresidence time is sufficient to yield the desired thickness ofcementation layer. The feed rate of hydrogen selected is sufficient tofluidize the bed, the silane concentration is 1-5% to produce low dustoperation.

The process of this invention produces a highly desirable polysiliconproduct useful for the production of silicon semiconductor devices. Theproduct in the form of approximately spherical particles. These are freeflowing, and therefore can be handled much more readily by mechanizedsystems than the rods formed by the Siemens process. Mechanical systemscan be designed for storage, and handling of the free flowing materialproduced by this invention which decrease the chances for contamination.In general the size distribution of products produced by this inventionhave the following size distribution (microns):

Typical range 150-1500

Typical average 650-750

The particle density (g/cc) is:

Typical range 2.25-2.33

Typical average 2.30-2.31

Preferred materials have a bulk density of about 1360 kg/m³. Surfacedust is typically 0.010-0.070 weight percent. Using silane of goodpurity the concentration of key transition metal impurities approachesor matches high quality available Siemens product. Typical purities fromoperation are:

    ______________________________________                                                            Average                                                                              Range                                              ______________________________________                                        Boron    ppba             0.12     0.01-0.25                                  Phosphorus                                                                             ppba             0.11     0.01-0.19                                           (parts per billion atomic)                                           Carbon   ppm              0.25     0.16-0.33                                  ______________________________________                                    

While the invention has been described in terms of various preferredembodiments, those skilled in the art will recognize that variouschanges, modifications or omissions, can be made without departing fromthe heart or spirit of the invention as described above and set forth inthe claims which follow below.

We claim:
 1. A semiconductor grade polysilicon in the form ofapproximately spherical particles having:(i) a surface morphologyillustrated by FIGS. 3 and 3A, (ii) a size distribution of from about400 to 1000 microns, (iii) an average size of 650 to 750 microns, (iv) aboron content within the range of 0.01 to 0.25 ppba, (v) a phosphoruscontent within the range of 0.01 to 0.19 ppba, (vi) a carbon contentwithin the range of 0.16 to 0.33 ppm, and (vii) a surface dust contentless than about 0.08 weight percent bonded to said particles by asilicon layer of 0.1 to about 5 microns thick;said semiconductor gradepolysilicon being free flowing and suitable for continuous meltreplenishment systems for producing monocrystalline silicon.
 2. Thepolysilicoon of claim 1 having a particle density of about 2.3 grams percentimeter.
 3. The polysilicon of claim 2 having a bulk density of about1360 kg/m³.
 4. The polysilicon of claim 1 in which said silicon layer is0.1 to 1.0 micron in thickness.
 5. the polysilicon of claim 1 whereinsaid silicon layer is produced by thermal decomposition of silane gas.